DTD (Document Type Definition)

A DTD is the definition of the allowable elements, attributes, and other document pieces of an XML document.

The DITA DTDs are (base) topic, concept, task, reference, map, and bookmap. Every DITA file must reference its DTD in the DOCTYPE declaration.

DTDs are not written in XML syntax.

The following figure shows a snippet of the DITA Concept DTD.

Related concepts
DOCTYPE declaration
XML (eXtensible Markup Language)